This invention relates to a torque detecting apparatus and, particularly, to a torque detecting apparatus which detects the torque from the torsion of a torsion bar. The invention is suitable for use with a torque assisting device including a steering force assisting system of a vehicle for example, and therefore the invention is not precluded from including the power steering system.
An example of a conventional torque measuring apparatus using a torsion bar is one invented by Parkinson, as described in Japanese Patent Publication No. 54-14949. Parkinson's torque detector includes a second gear (14) fitted directly on a shaft (13) and a first gear (11) fitted at the free end of a sleeve (12) which is secured to the shaft (13). The apparatus further includes a first magnetic transducer (16) and third magnetic transducer (17) for detecting teeth of the first gear and second magnetic transducer (17) and fourth magnetic transducer (20) for detecting teeth of the second gear, and operates to measure the torque from the torsion of shaft which is determined from the difference of rotational positions of the first and second gears detected by the sensors. However, it is not clear what kind of magnetic transducers are used in the Parkinson invention. The description that the magnetic transducers are coupled with the gears, and the associated FIG. 3 suggest the transducers to be of the electromagnetic pickup coil type.
Transducers of the electromagnetic pickup coil type have their output signals dependent in magnitude on revolutions of the gears. The transducer produces a smaller output when the gear rotates slowly, and conceivably it is not capable of detecting the torque when the gear rotates very slowly or is quiescent.
A torque measuring apparatus using a torsion bar is described in Japanese Utility Model Laid-Open No. 61-76338 by Yoneda. The apparatus includes two magnetic media (24, 25) fitted with a certain spacing on a torque transmission shaft (21), and their magnetic poles N and S are detected by magnetic resistance elements (28, 29). The magnetic resistance element has its internal resistance varied in response to the magnetic field, and is operative to detect magnetic poles even if the magnetic media are quiescent.
Although Yoneda's invention overcomes the problem of the above-mentioned Parkinson invention, it is designed to measure the torque through the computation of pulses derived from the signals of the magnetic resistance elements (28, 29) (refer to FIG. 1) and therefore it cannot measure the torque if pulses are not produced in a number suitable for computation. Specifically, it is infeasible to detect a small variation of torque occurring between contiguous pulses.